The segment routing module adds to the TTM a shortest path SR tunnel entry for each resolved remote node SID prefix and programs the datapath with the corresponding LTN with the push operation pointing to the primary and LFA backup NHLFEs. The LFA backup next-hop for a prefix that was advertised with a node SID is computed only if the loopfree-alternate option is enabled in the IS-IS or OSPF instance. The resulting SR tunnel that is populated in the TTM is automatically protected with FRR when an LFA backup next-hop exists for the prefix of the node SID.
With ECMP, a maximum number of primary next-hops (NHLFEs) are programmed for the same tunnel destination per IGP instance. ECMP and LFA next-hops are mutually exclusive as per the existing implementation.
The 7210 SAS-Mxp, 7210 SAS-R6, 7210 SAS-R12, and 7210 SAS-Sx/S 1/10GE support only LSR ECMP; LER ECMP is not supported on these platforms.
The default preference for shortest path SR tunnels in the TTM is set lower than LDP tunnels but higher than BGP tunnels to allow controlled migration of customers without disrupting their current deployment when they enable segment routing. The following list presents the default preference of the various tunnel types. This includes the preference of both SR tunnels based on shortest path (referred to as SR-ISIS and SR-OSPF).
The global default TTM preference for the tunnel types is as follows:
ROUTE_PREF_RSVP 7
ROUTE_PREF_SR_TE 8
ROUTE_PREF_LDP 9
ROUTE_PREF_OSPF_TTM 10
ROUTE_PREF_ISIS_TTM 11
ROUTE_PREF_BGP_TTM 12
ROUTE_PREF_GRE 255
The default value for SR-ISIS or SR-OSPF is the same regardless of whether one or more IS-IS or OSPF instances are programming a tunnel for the same prefix. The selection of an SR tunnel in this case is based on the lowest IGP instance.
The TTM preference is used in the case of VPRN auto-bind or BGP transport tunnels when the tunnel binding commands are configured to the any value, which parses the TTM for tunnels in the protocol preference order. The user can choose to either use the global TTM preference or list explicitly the tunnel types to be used. When the tunnel types are listed, the TTM preference is still used to select one type over the other. In both cases, a fallback to the next preferred tunnel type is performed if the selected one fails. Also, a reversion to a preferred tunnel type is performed as soon as one is available. See BGP label route resolution using segment routing tunnels and Service packet forwarding with segment routing for the detailed service and shortcut binding CLI.
For SR-ISIS and SR-OSPF, the user can configure the preference of each IGP instance in addition to the preceding default values.
configure>router>isis>segment-routing>tunnel-table-pref preference <1..255>
configure>router>ospf>segment-routing>tunnel-table-pref preference <1..255>SR tunnels in the TTM are available to BPG routes, VPRN auto-bind and explicit SDP binding, and L2 services with PW template auto-bind and explicit SDP binding.
Local adjacency SIDs are not programmed into the TTM, but the remote adjacency SIDs can be used together with a node SID in a tunnel configuration in a directed LFA.